1. Technical Field of the Invention
The present invention relates to a precision aircraft landing aid, more particularly to a laser landing altimeter.
2. Prior Arts
Landing is the most challenging part of flying. For light aircrafts, a normal landing consists of two maneuvers: flare and pre-touchdown. When the aircraft flies into the ground effect, a pilot initiates a first pitch change so that the aircraft flies level above the ground. As the aircraft loses speed and becomes sufficiently close to the ground, the pilot initiates a second pitch change to reduce the descent rate at touchdown. Hereinafter, the first pitch change is referred to as flare, the time and altitude to initiate flare are referred to as flare time and flare altitude, respectively. The second pitch change is referred to as pre-touchdown, and the time and altitude to initiate pre-touchdown are referred to as pre-touchdown time and pre-touchdown altitude, respectively. Overall, the flare time and pre-touchdown time are collectively referred to as landing maneuver time, while the flare altitude and pre-touchdown altitude are collectively referred to as landing maneuver altitude. The same definitions apply to both small and large aircrafts.
For small aircrafts, the flare altitude is typically ˜5 m to ˜10 m above ground level (AGL). Student pilots generally have difficulty judging the flare altitude and need to practice hundreds of landings before getting to know when to flare. Practicing such a large number of landings lengthens the training time, wastes a large amount of fuel and has a negative impact to the environment. Although a radio altimeter may be used to help flare, it is expensive. A low-cost landing aid is needed for student pilots to master landing skills quickly and with relative ease.
For small aircrafts, the pre-touchdown altitude is typically less than ˜2 m AGL, preferably ˜1 m AGL. It can tolerate much less error than the flare altitude. Most pilots, even experienced pilots, have difficulty judging the pre-touchdown altitude. Because a radio altimeter only has an accuracy of ±˜1 m, it cannot be used to help pre-touchdown. In order to make a gentle touchdown, a precision landing aid is desired to precisely measure the altitude when the aircraft is near the ground, preferably with centimeter (cm) accuracy.
For large aircrafts, standard landing procedure teaches a single maneuver: flare. At the flare altitude, a pilot initiates a pitch change to reduce the descent rate and holds the flare pitch until the aircraft flies onto the runway. During flare, an intermediate descent rate (e.g. ˜2 m/s) is recommended. However, this intermediate descent rate may cause disturbance to the passengers at touchdown. To improve passenger comfort, a certain degree of pre-touchdown maneuver is preferably performed to reduce the descent rate at touchdown. This requires a precision landing aid, which can precisely measure the aircraft altitude when the aircraft is near the ground, preferably with centimeter (cm) accuracy.
U.S. Pat. No. 7,106,424 issued to Meneely et al. on Sep. 12, 2006 and U.S. Pat. No. 7,400,386 issued to Jamieson et al. on Jul. 15, 2008 disclose a pulsed laser altimeter. It directly measures the time for a short laser pulse to travel from the laser source to a remote object and then back to the laser source, i.e. time-of-flight (TOF). The pulsed laser altimeter has a range of up to several kilometers (km) and an accuracy of ±˜1 m. Similar to a radio altimeter, this accuracy is not good enough for precision landing aid.
U.S. Pat. No. 6,864,966 issued to Giger on Mar. 8, 2005, U.S. Pat. No. 5,309,212 issued to Clark on May 3, 1994 and U.S. Pat. No. 4,611,912 issued to Falk et al. on Sep. 16, 1986 disclose several laser distance meters (LDM). They measure distance using a modulated laser beam. The LDM has a range of tens of meters and an accuracy of millimeter (mm). To achieve the mm accuracy, an LDM statistically evaluates hundreds to thousands of distance data. The evaluation period T (i.e. the time it takes to generate a new distance reading) is long, with a typical value of ˜0.1 s to ˜7 s.
The LDM is designed to measure static distance, i.e. distance to a stationary (or, slow-moving) object. It is not designed to measure dynamic distance, i.e. distance to a fast-moving object. For a fast-moving object, the real-time distance is not as useful as the predicted future distance (e.g. the aircraft altitude at a future time). Unfortunately, the LDM does not have the capability to extract this information. Furthermore, it has a long evaluation period, which makes it virtually impossible to perform a meaningful distance measurement for a landing aircraft. As illustrated in FIG. 1, a landing aircraft has a speed of ˜60 knots (˜30 m/s) to ˜230 knots (˜115 m/s) and a descent rate of ˜1.5 m/s to ˜6 m/s. With a typical evaluation period T (—0.5 s), the flying distance L is from ˜15 m to ˜58 m and the altitude loss ΔA (=A1−A2) is from ˜0.8 m to ˜3 m. Over such a long distance L, any foreign object (e.g. an ILS antenna at location o) located under the approach path will cause a large fluctuation to the distance data. In addition, the large altitude loss ΔA due to landing will certainly trigger an error. Hence, the LDM is not suitable for precision landing aid.